I apologize for my naive question. However, I think it's good way to ask my question to linux-kernel gurus here.

Basically, I want to add a system call which return the window size of the corresponding tcp connection. It should have a parameter of socket descriptor. (Say, get_winsize( int sock ) )

My question is which in-kernel data structure has the window size value of the ongoing tcp connection and how can I get that value from the socket descriptor. Actually, the question is related with the linkage of data structures from the socket to the tcp window size.